encoding/base64: fix typo in comment
encoding/base64: fix typo in comment

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/258041

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,ssrl/go,nightlyone/golang,h8liu/golang,gnanderson/go,bdwalton/debian-golang,tianon/debian-golang,bdwalton/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,ssrl/go,nightlyone/golang,tv42/old-go,bdwalton/debian-golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,tianon/debian-golang,h8liu/golang,bdwalton/debian-golang,tianon/debian-golang,ssrl/go,nightlyone/golang,bdwalton/debian-golang,tv42/old-go,h8liu/golang,apparentlymart/golang,nightlyone/golang,gnanderson/go,ssrl/go,nightlyone/golang,apparentlymart/golang,dsriram/golang,ssrl/go,tianon/debian-golang,bdwalton/debian-golang,gnanderson/go,bdwalton/debian-golang,apparentlymart/golang,tianon/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,h8liu/golang,apparentlymart/golang,h8liu/golang,gnanderson/go,nightlyone/golang,h8liu/golang,nightlyone/golang,bdwalton/debian-golang,ssrl/go,ssrl/go,tv42/old-go,pombredanne/go-deleteme,h8liu/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,tianon/debian-golang